Ceramic glaze having antimicrobial property

ABSTRACT

An antimicrobial ceramic glazing composition contains one or more antimicrobial agents disposed therein. Methods for making and using the glazing composition are disclosed, as well as substrates having a fired antimicrobial glaze thereon. The antimicrobial agents comprise metallic oxides, with a subset of the disclosed combinations exhibiting synergistic effect in fired glazes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/931,805, filed Jun. 28, 2013, which is a divisional of U.S.application Ser. No. 12/032,657, filed Feb. 16, 2008, which claimspriority from U.S. provisional application 60/890,673, filed Feb. 20,2007, and from U.S. provisional application 60/890,666, filed Feb. 20,2007, the contents of which are incorporated by reference in theirentireties as though fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to the field of antimicrobial protectionin a ceramic article or component thereof. More specifically, thepresent invention relates to a composition for imparting built-in andlong lasting antimicrobial characteristics to ceramic products.

BACKGROUND OF THE INVENTION

An area of particular commercial interest in the art is ceramic articlesand ceramic coatings. Ceramic coatings are commonly used in productsthat store, treat, or transport water and liquid waste. Ceramic toilets,urinals, bidets, bathroom basins (collectively known as sanitary ware),flooring tiles and other bathroom fixtures are probably the most commonexample of such products.

When used to collect, contain and/or transport water, ceramic productsoften become stained by scum and films of biologic origin (e.g.,bacteria, fungus, mold, mildew). To date, the primary method of removingbiological scum and film from these ceramic products has been to abradethe ceramic surface in the presence of a topical cleaning agent.

There is a need for a ceramic coating that has built-in protectionagainst the growth and proliferation of microbes. However, existingtechnologies are somewhat limited in this regard. For example, the hightemperatures used in ceramic firing processes typically preclude the useof organic antimicrobial agents.

Conventional inorganic silver-based antibacterial compounds (e.g.,zeolite, amorphous glass, sol-gel) generally are too expensive forcommercial use. Moreover, incorporation of silver-based antimicrobialagents into ceramic glazes routinely presents issues of clouding,crazing, discoloration, and other undesirable consequences to the glazeaesthetics.

Zinc oxide is known as having antimicrobial characteristics and has beenused in the preparation of ceramic glazing compositions. However, knownceramic glazing compositions that rely solely upon zinc oxide as anantimicrobial agent have not shown antimicrobial efficacy sufficient forcontrol of microbial growth and proliferation on ceramic surfaces.

Accordingly, there is a need for a low-cost ceramic coating that offerspersistent built-in antimicrobial protection.

DETAILED DESCRIPTION

As used herein, the terms “microbe” or “microbial” should be interpretedto refer to any of the microscopic organisms studied by microbiologistsor found in the use environment of a ceramic article or ceramic-glazedarticle. Such organisms include, but are not limited to, bacteria andfungi as well as other single-celled organisms such as mold, mildew andalgae. Viral particles and other infectious agents are also included inthe term microbe.

As well, “antimicrobial” and like terms should be interpreted asencompassing both microbe-killing as well as microbistatic activities.That is, it herein is considered efficacious if an antimicrobialcomposition reduces the number of microbes on a substrate or if thecomposition retards the normal rate of microbial growth.

For ease of discussion, this description uses the terms microbes andantimicrobial to denote a broad spectrum activity (e.g. against bacteriaand fungi). When speaking of efficacy against a particular microorganismor taxonomic rank, the more focused term will be used (e.g. antifungalto denote efficacy against fungal growth in particular).

Using the above example, it should be understood that efficacy againstfungi does not in any way preclude the possibility that the sameantimicrobial composition demonstrates efficacy against another class.

For example, discussion of the strong bacterial efficacy demonstrated bya disclosed embodiment should not be read to exclude the embodiment fromalso demonstrating antifungal activity. This method of presentationshould not be interpreted as limiting the scope of the invention in anyway.

A first embodiment is an antimicrobial ceramic glaze composition. Asecond embodiment disclosed herein is a method for making anantimicrobial ceramic glazing composition. The glaze compositioncomprises a plurality of conventional glaze ingredients and acombination of antimicrobial agents, as described more fully below.

The following brief discussion of ceramic coatings and in particular toceramic glazing on the outer surfaces of ceramic products and ofvitreous china or ceramic production is provided as an aid to thereader. This discussion is presented in the context of the production ofbathroom fixtures. Those skilled in the art recognize that theproduction process of ceramic products may vary from that which ispresented below, and that the ceramic glazing process disclosed hereinis adaptable to other substrates.

Glazes are generally made from powdered glass combined with coloredoxides of such elements as cobalt, chrome, manganese, or nickel. Thepowder mixture is suspended in water and applied to the ceramic surfaceby spraying, brushing, dipping, or other known application methods.

The suspension, or slip, in which the glaze is applied to the ceramicsurface must have particular properties to ensure that the glaze is easyto apply, does not run during firing, and adheres well both when wet andafter firing. These slip properties are often obtained by adding a smallamount of clay to the suspension and by controlling both the amount ofwater in the slip as well as the size of the powder particles. Organicsurface-active agents (e.g. surfactants, detergents) also can be addedto the slip to improve its properties.

Colors in glazes are controlled by adding coloring agents to the glassycomponents of the glaze. Special effects in glazes can also be produced.If salt is added to the kiln during firing, the glaze develops a fineorange-peel texture, which can be uniform or spotty depending uponconditions. A glaze that froths during firing gives a rough surface ofbroken bubbles known as a blister glaze.

It was undertaken to develop a baseline glaze composition—that is, aconventional, non-antimicrobial ceramic glaze base—and methodology toaid in identification of suitable and efficacious antimicrobial agents.By way of specific technical background, development of the baselineglaze, its processing, and its maturing temperature are now brieflyreviewed.

Two potential glaze frits were identified that did not contain any ofthe antimicrobial agents that could comprise at least 95% of the glazecomposition. These frits were used to constitute glaze slips that wereapplied to bisque-fired tile. The slips were evaluated at differentsolids contents and viscosities. It is also preferable that the glazesbe easy to apply by multiple methods.

A conventional glaze composition used for experimental trials herein iscomposed of 95% slow-fire base glaze (containing primarily SiO₂ andsecondarily, inter alia, KNaO, CaO, BaO, SrO, Al₂O₃ and B₂O₃).

Alkaline earth oxide materials such as calcium carbonate, wollastonite,and zinc oxide are generally added as raw materials. Other alkalineearth oxides such as lead oxide, strontium oxide, barium oxide, andmagnesium oxide are more typically added in a fritted form. The alkalineearth oxides are advantageous because they provide fluxing actionwithout having a major effect on glaze thermal expansion. Oxides alsocan serve as coloring compounds.

Also resident in the glaze composition is 5% EPK kaolin, and anover-addition of 1% Bentonite (an absorbent aluminum silicate clayformed from volcanic ash and well known to those of skill in the ceramicart). This dry material is blended into a sufficient quantity ofde-ionized water to produce a glaze slip with a specific gravity of1.35±0.05 g/cc. This represents a solids content of 41.74%.

Notable among the many antimicrobial agents used were Ag₂CO₃ (CAS No.534-16-7); Bi₂O₃ (CAS No. 1304-76-3); CuO (CAS No. 1317-38-0); SnO₂ (CASNo. 18282-10-5); TiO₂ (anatase; CAS No. 13463-67-7); and ZnO (CAS No.1314-13-2).

An antimicrobial glazing composition was made by adding together (e.g.by mixing) conventional glazing composition components and antimicrobialagent combinations. The components and antimicrobial agent(s) were addedbased on weight of the solids content of the baseline glaze, excludingsuch antimicrobial agent(s). The glaze base is described in greaterdetail above.

The glaze base, with antimicrobial agent(s) admixed therein, then wasball milled for fifteen minutes. The milled glaze base was heldovernight to allow for hydration, then remixed. The antimicrobialglazing composition then was ready to be applied to a substrate (e.g. abisque tile).

All material addition calculations are based upon the percent solids ofthe baseline glaze and the specific gravity is checked before each groupof material evaluation samples is processed. Each material to beevaluated is added to 1000 milliliters of baseline glaze.

It is expected, however, that other conventional ceramic glazecompositions could be substituted without departing from the essentialfeatures of the antimicrobial ceramic glaze as described herein.

In a third embodiment, a method of affixing a ceramic glaze to asubstrate confers durable antimicrobial properties to the substrate. Themethod generally comprises providing a ceramic glazing compositionhaving one or more antimicrobial agents disposed therein as set forth inthe present disclosure, applying the antimicrobial glazing compositionto a substrate, and curing the glazing composition in accordance withconventional glaze-firing techniques.

Development work utilized dipping to apply the glaze formulation totiles, although other methods of application known to those in the artmay be used. The glaze is then dried and fixed onto the ceramic surfaceby firing.

During firing, the powdered glass softens and largely equilibrates overthe ceramic surface, reacting with the ceramic substrate to form astrong, adherent union therewith. If a glaze is applied to an alreadyfired ceramic substrate, a second firing is necessary to melt and bondthe glaze to the substrate. Alternatively, it is possible to apply aglaze to an unfired ceramic and fire both the glaze and substratetogether.

Various components, such as alkali oxide, borates, and lead oxide can beadded to the ceramic glaze composition to facilitate softening at lowertemperatures in order that the glaze flow more easily during firing andto minimize roughness and defects in the fired ceramic glaze surface.The present antimicrobial combinations are compatible with these commonadditives.

The initial stage of a typical ceramic production process is theproduction of barbotine or slip, a clay from which bathroom ceramicproducts are made. Barbotine is made from a mixture of clays, kaolin,phyllites, feldspar and quartz.

Individual pieces are cast by pouring the barbotine into molds made ofgypsum or microporous resin. In the casting processes that use gypsummolds, the parts are formed by absorption of water contained in thebarbotine through the capillary action of the gypsum. As water leavesthe barbotine the part solidifies to a point where the mold can beopened. The still malleable part is then removed from the mold.

Casting processes that use resin molds are called “high pressure”processes. Parts are formed by filtering water contained in thebarbotine clay through micropores in the resin molds by the applicationof pressure. The water is eliminated by injecting compressed air alongthe molds.

After casting and removal from the molds, the parts go for drying inkilns under controlled humidity and temperature (approximately90.degree. C.). The drying cycle lasts about 7 hours, reducing the watercontent of the part from about 16% to less than 1%. Following this, theparts are inspected to detect possible flaws. The parts then go to thecoating process. The coating process is alternatively referred to as theglazing step.

The glazing step typically comprises the application of ceramic glaze onthe parts using guns in individual booths fitted with exhaust systemsand water curtains. Typical ceramic glaze is produced from a mixture ofkaolin, feldspar, quartz, colorings and other additives. Once coated,the parts are fired in continuous kilns, reaching temperatures of about1250° C. in an approximately 15-hour cycle. The firing process gives theglazed part the color and transparent appearance that is typical ofvitreous china.

The procedure to manufacture material evaluation samples isstraightforward. A reservoir of baseline glaze, as described above, ismaintained. Sample tiles have applied thereon or thereto the presentglaze composition; the present disclosure relates to dipped sampletiles.

Each dipped tile was placed into a tile sagger, each sagger capable ofholding up to twenty tiles. The sagger was placed into one of the twoelectric kilns and fired to a Pyrometric Cone Equivalent of 06. Thismeasure of thermal history is roughly equivalent to 1889° F. or 1062° C.Baseline glaze samples were fired at temperatures ranging from 1888° F.to 2194° F.

The above procedure approximates a glaze application in a productionenvironment. The final baseline glaze formulations resulted in sampleshaving a glassy surface at a low temperature, highly resistant toabsorption of dyes, and exhibiting no antimicrobial properties.

Microscopic imaging of the baseline glaze/tile interface revealedcomplete vitrification of the glaze with no inclusions of bubbles orunmelted materials. The baseline samples are the foundation forcomparing and judging the candidate materials. The adopted baselineglaze is simple in composition, easy to process and apply, and has a lowfiring temperature. These attributes greatly facilitated the evaluationof candidate material samples.

In production, this dried glaze layer is about 2 millimeters inthickness. A more cost efficient method of producing an antimicrobialsurface would entail use of a much thinner secondary glaze applied overthe regular (first) glaze. This glaze thickness could be 0.5 millimetersor less in thickness.

It is expected that exposure of the glazed tile to microbes will resultin microbial contact only with the glazed composition on the surface ofthe glazed tile. Material below the surface is trapped within the glassof the glaze and thereby sequestered from microbes.

A variety of antimicrobial agents were tested in the baseline glazecomposition after glazing of a substrate sample. Of these compounds, avariety of combinations also were assessed, as detailed in the followingdiscussion and examples.

In a fourth embodiment, a ceramic article bearing the above-describedantimicrobial glaze composition exhibits durable antimicrobialproperties. The antimicrobial ceramic article comprises a substrate, forexample a ceramic substrate, having at least a first surface; and afired or cured glaze disposed on at least a portion of the firstsurface. The ceramic glazing composition utilized in this embodiment isthe same as that described in the first embodiment.

Antimicrobial agents were used to manufacture a range of antimicrobialglaze compositions, each composition consisting of one, two or threeantimicrobial agents. Several ceramic articles then were prepared totest the antimicrobial characteristics of the recited glazes. The testarticles comprised an underlying ceramic substrate made from a standardcommercial barbotine.

The glaze used in the testing was the baseline glaze describedpreviously, to which was added varying quantities of antimicrobial agentcombinations as noted. The glaze composition was applied to the articlesby dipping, and the test articles were then fired.

As mentioned, combinations of two antimicrobial agents were assessed forantimicrobial efficacy in fired glazes on ceramic substrates. Thecompounds Ag₂CO₃, Bi₂O₃, CuO, SnO₂, TiO₂, and ZnO were evaluated. Eachcompound was sequentially trialed at 2% in tandem with one of the otherfive compounds. The second compound was trialed at either 2% or 4%. Asan example using Ag₂CO₃ and ZnO, then, the following possibilities weretested: 2% Ag₂CO₃ and 2% ZnO; 2% Ag₂CO₃ and 4% ZnO; and 4% Ag₂CO₃ and 2%ZnO.

Continuing with this exemplary combination of antimicrobial agents, thecombination of 4% Ag₂CO₃ and 4% ZnO was not tested, as 4%/4%combinations generally are considered too expensive to be commercializedand/or are have been observed to negatively affect the aesthetics of theglaze finish. It is expected that such combinations would show efficacyif the 2%/4% and/or 4%/2% combination was efficacious, althoughantagonistic effects have been observed for some combinations.

Test articles also were prepared without any antimicrobial agents in theglaze for use as a negative control.

The measure of antimicrobial efficacy is the reduction in the number oforganisms surviving the testing protocol in comparison with the baselinestandard. Minimum efficacy is assumed to originate at a reduction levelof 1 common logarithm (log (NOS Std/NOS Sample)).

Three samples of each addition level and three baseline glaze sampleswere then tested in triplicate. The testing is in accordance with amodified JIS Z2801:2000 test protocol (available from JapaneseIndustrial Standards Committee, Tokyo, Japan). The Z2801 protocol is aninternationally known standard test for antimicrobial activity andefficacy. The protocol and specific modifications made thereto aresummarized below.

Sample tile pieces having a diameter of approximately 55 mm were used.Ceramic glaze composition was applied and fired according to theinstructions for the commercial glaze base employed. This preparationprocess yielded test disks having about a top surface area of about 2500square millimeters.

The comparison test for antimicrobial efficacy used Klebsiellapneumoniae, ATCC 4352. The test organism was grown, and a portion of anexponentially growing culture was collected into Japanese Nutrient Broth(JNB) diluted 1/500. An inoculum was prepared at about 10⁶colony-forming units (CFU) per milliliter by dilution with 1/500 JNB.

A sample tile was placed on moistened laboratory tissue in a cultureplate, and 75 microliters of test inoculum (˜0.8×10⁵ CFU) was pipettedonto the sample surface. A cover slip or film was placed over and incontact with the inoculum to ensure uniform and substantially completecoverage of the inoculum over the sample surface. The culture plate thenwas incubated for 24 hours at 37° with humidity.

Bacteria on the sample and cover slip/film were recovered, collectedinto D/E Neutralizing Broth, and counted. The antimicrobial activity ofthe test samples is expressed herein as a log reduction value, ascompared to the bacterial growth of the corresponding untreated(control) sample. A log reduction is expressed as log (U/B), where U isthe average CFU of the test organism from the inoculum recovered in theNeutralizing Broth from the negative control (untreated) sample tile,and B is the average CFU of the test organism recovered in theNeutralizing Broth from the inoculated sample.

EXAMPLE 1

In a first example, 2% Ag₂CO₃ was utilized as a first antimicrobialagent in a family of glaze compositions, which further contained asecond antimicrobial agent: one of Bi₂O₃, CuO, SnO₂, TiO₂ or ZnO. Thesecond antimicrobial agent was tested at both 2% and 4%. Sample tileswere glazed and the tiles were evaluated according to theabove-described modified JIS Z2801:2000 test protocol for the effect ofthe glazed tile on bacterial reduction. Results are shown in TABLE 1.

EXAMPLE 2

In a second example, 2% Bi₂O₃ was utilized as a first antimicrobialagent in a family of glaze compositions, which further contained asecond antimicrobial agent: one of Ag₂CO₃, CuO, SnO₂, TiO₂ or ZnO. Thesecond antimicrobial agent was tested at both 2% and 4%. Sample tileswere glazed and the tiles were evaluated according to theabove-described modified JIS Z2801:2000 test protocol for the effect ofthe glazed tile on bacterial reduction. Results are shown in TABLE 2.

TABLE 1 K. Pneumoniae Antimicrobial Combination: Ag₂CO₃ Log Ag₂CO₃ Bi₂O₃CuO SnO₂ TiO₂ ZnO reduction — — — — — — NA 2% — — — — — 2.4 2% 2% — — —— 3.6 2% 4% — — — — 3.0 2% — 2% — — — 3.9 2% — 4% — — — 3.8 2% — — 2% —— 3.2 2% — — 4% — — 1.9 2% — — — 2% — 2.4 2% — — — 4% — 2.7 2% — — — —2% 3.0 2% — — — — 4% 3.3

EXAMPLE 3

In a third example, 2% CuO was utilized as a first antimicrobial agentin a family of glaze compositions, which further contained a secondantimicrobial agent: one of Ag₂CO₃, Bi₂O₃, SnO₂, TiO₂ or ZnO. The secondantimicrobial agent was tested at both 2% and 4%. Sample tiles wereglazed and the tiles were evaluated according to the above-describedmodified JIS Z2801:2000 test protocol for the effect of the glazed tileon bacterial reduction. Results are shown in TABLE 3.

TABLE 2 Antimicrobial Combination: Bi₂O₃ K. Pneumoniae Bi₂O₃ Ag₂CO₃ CuOSnO₂ TiO₂ ZnO Log reduction — — — — — — NA 2% — — — — — 0.8 2% 2% — — —— 1.3 2% 4% — — — — 3.7 2% — 2% — — — 1.9 2% — 4% — — — 3.1 2% — — 2% —— 0.5 2% — — 4% — — 0.9 2% — — — 2% — −0.2 2% — — — 4% — 0.8 2% — — — —2% 0.7 2% — — — — 4% 1.4

TABLE 3 Antimicrobial Combination: CuO K. Pneumoniae CuO Ag₂CO₃ Bi₂O₃SnO₂ TiO₂ ZnO Log reduction — — — — — — NA 2% — — — — — 0.4 2% 2% — — —— 3.8 2% 4% — — — — 4.0 2% — 2% — — — 2.5 2% — 4% — — — 1.9 2% — — 2% —— 0.3 2% — — 4% — — 2.4 2% — — — 2% — 2.0 2% — — — 4% — 2.3 2% — — — —2% 0.5 2% — — — — 4% 3.0

EXAMPLE 4

In a fourth example, 2% SnO₂ was utilized as a first antimicrobial agentin a family of glaze compositions, which further contained a secondantimicrobial agent: one of Ag₂CO₃, Bi₂O₃, CuO, TiO₂ or ZnO. The secondantimicrobial agent was tested at both 2% and 4%. Sample tiles wereglazed and the tiles were evaluated according to the above-describedmodified JIS Z2801:2000 test protocol for the effect of the glazed tileon bacterial reduction. Results are shown in TABLE 4.

TABLE 4 Antimicrobial Combination: SnO₂ K. Pneumoniae SnO₂ Ag₂CO₃ Bi₂O₃CuO TiO₂ ZnO Log reduction — — — — — — NA 2% — — — — — 0.1 2% 2% — — — —0.5 2% 4% — — — — 3.7 2% — 2% — — — 1.2 2% — 4% — — — 0.5 2% — — 2% — —0.5 2% — — 4% — — 4.0 2% — — — 2% — 0.0 2% — — — 4% — 0.1 2% — — — — 2%0.2 2% — — — — 4% 0.4

TABLE 5 Antimicrobial Combination: TiO₂ K. Pneumoniae TiO₂ Ag₂CO₃ Bi₂O₃CuO SnO₂ ZnO Log reduction — — — — — — NA 2% — — — — — 0.0 2% 2% — — — —0.8 2% 4% — — — — 3.6 2% — 2% — — — 0.7 2% — 4% — — — 0.7 2% — — 2% — —0.4 2% — — 4% — — 3.9 2% — — — 2% — 0.0 2% — — — 4% — 0.5 2% — — — — 2%0.0 2% — — — — 4% 0.2

EXAMPLE 5

In a fifth example, 2% TiO₂ was utilized as a first antimicrobial agentin a family of glaze compositions, which further contained a secondantimicrobial agent: one of Ag₂CO₃, Bi₂O₃, CuO, SnO₂ or ZnO. The secondantimicrobial agent was tested at both 2% and 4%. Sample tiles wereglazed and the tiles were evaluated according to the above-describedmodified JIS Z2801:2000 test protocol for the effect of the glazed tileon bacterial reduction. Results are shown in TABLE 5.

EXAMPLE 6

In a sixth example, 2% ZnO was utilized as a first antimicrobial agentin a family of glaze compositions, which further contained a secondantimicrobial agent: one of Ag₂CO₃, Bi₂O₃, CuO, SnO₂ or TiO₂. The secondantimicrobial agent was tested at both 2% and 4%. Sample tiles wereglazed and the tiles were evaluated according to the above-describedmodified JIS Z2801:2000 test protocol for the effect of the glazed tileon bacterial reduction. Results are shown in TABLE 6.

TABLE 6 Antimicrobial Combination: ZnO K. Pneumoniae ZnO Ag₂CO₃ Bi₂O₃CuO SnO₂ TiO₂ Log reduction — — — — — — NA 2% — — — — — 0.4 2% 2% — — —— 2.4 2% 4% — — — — 3.7 2% — 2% — — — 1.3 2% — 4% — — — 1.5 2% — — 2% —— 0.5 2% — — 4% — — 3.7 2% — — — 2% — 0.2 2% — — — 4% — 0.2 2% — — — —2% −0.1 2% — — — — 4% 0.1

Results

When two chemical antimicrobial agents are used in combination, eitherin a single composition or as two separate additions at the point ofuse, three results are possible: 1) an additive (neutral) effect; 2) anantagonistic effect; or 3) a synergistic effect.

An additive (neutral) effect has no economic advantage over theindividual antimicrobial agents. An antagonistic effect would produce anegative or reduced-efficacy result.

Only synergism, which is much less likely than an additive or anantagonistic effect, gives a positive result and, therefore possesseseconomic advantages.

According to the invention, the combinations identified belowdemonstrate an unexpected and synergistic antimicrobial effect in afired ceramic glaze. The combinations of first and second antimicrobialagents, as described herein, achieve superior antimicrobial activity atlower antimicrobial agent concentrations as compared to theantimicrobial capability of either antimicrobial agent alone. Such asuperior effect presents a distinct economic advantage and increases theeffectiveness of the antimicrobial combination per unit weight.

Looking at the results for antimicrobial agents singly and incombinations where the first antimicrobial agent is 2% Ag₂CO₃ (resultsin TABLE 7), it can be seen that a 2% addition of one antimicrobialagent alone demonstrated a range of efficacy results: Ag₂CO₃ (2.4;efficacy), Bi₂O₃ (0.8; weak efficacy), CuO (0.4; very weak efficacy),ZnO (0.4; very weak efficacy), SnO₂ (0.1; essentially no efficacy) andTiO₂ (0.0; no efficacy).

However, it readily can be seen that simple additions of a secondantimicrobial agent at 2% resulted in any one of additive, antagonistic,or synergistic effect. Moreover, addition of the second antimicrobialagent at 4% did not generate results in keeping with expectations basedon the test results of individual antimicrobial agents or [2%+2%]antimicrobial agent combinations.

For combinations of Ag₂CO₃ and Bi₂O₃, the [2% Ag₂CO₃+2% Bi₂O₃]combination exhibits a synergistic effect relative to the results to beexpected.

However, it is noted that twice the level of the second antimicrobialagent (that is, [2% Ag₂CO₃+4% Bi₂O₃]) displays an antagonistic effect,wherein the observed efficacy is lower than both (a) the 30 expectedadditive log reduction value for [2% Ag₂CO₃+4% Bi₂O₃]; and (b) theobserved log reduction value for the [2% Ag₂CO₃+2% Bi₂O₃] combination.

TABLE 7 Antimicrobial Combination: ZnO K. Pneumoniae Ag₂CO₃ Bi₂O₃ CuOSnO₂ TiO₂ ZnO Log reduction — — — — — — NA 2% — — — — — 2.4 — 2% — — — —0.8 2% 2% — — — — 3.6 2% 4% — — — — 3.0 2% — — — — — 2.4 — — 2% — — —0.4 2% — 2% — — — 3.9 2% — 4% — — — 3.8 2% — — — — — 2.4 — — — 2% — —0.1 2% — — 2% — — 3.2 2% — — 4% — — 1.9 2% — — — — — 2.4 — — — — 2% —0.0 2% — — — 2% — 2.4 2% — — — 4% — 2.7 2% — — — — — 2.4 — — — — — 2%0.4 2% — — — — 2% 3.0 2% — — — — 4% 3.3

For combinations of Ag₂CO₃ and CuO, the [2% Ag₂CO₃+2% CuO] combinationexhibits a strong synergistic effect relative to the expected resultsbased on merely additive principles. Increasing the level of the secondantimicrobial agents two-fold (that is, [2% Ag₂CO₃+4% CuO]) destroys thesynergistic effect, instead resulting in antagonism: the observedefficacy of the [2% Ag₂CO₃+4% CuO] combination (3.8) is essentially thesame as the [2% Ag₂CO₃+2% CuO] combination (3.9/3.8) and well below thevery high expected log reduction for this combination.

Ag₂CO₃ and SnO₂ demonstrated surprising and strong synergy for the [2%Ag₂CO₃+2% SnO₂] combination (3.2 log reduction). Unexpectedly, thecombination of Ag₂CO₃ and SnO₂ showed marked antagonism when the SnO₂concentration was doubled to 4%: log reduction tumbled to 1.9, wellbelow both the observed result of 3.2 for the [2% Ag₂CO₃+2% SnO₂]combination as well as the expected additive result.

The results for 2% Ag₂CO₃ and 2% TiO₂ were concluded to be merelyadditive. Unexpectedly, though, doubling the TiO₂ to 4% resulted in aminor synergistic effect: the efficacy of the [2% Ag₂CO₃+4% TiO₂]combination (2.7 log reduction value) was slightly above both theexpected additive-effect value for the combination and the observed logreduction for the [2% Ag₂CO₃+2% TiO₂] combination.

The assessment of Ag₂CO₃+ZnO combinations showed marginal synergy forthe [2% Ag₂CO₃+4% ZnO] combination (3.0 observed log reduction value).The synergy was lessened with the increase in the ZnO concentration to4% (3.3 actual).

The data presented for the other two-compound antimicrobial combinationslikewise can be analyzed, and further examples identified of additive,synergistic, and antagonistic effect.

A number of combinations were deemed to be of special interest. Thesecombinations are listed in TABLE 8 as showing synergistic effects. Thatis, the observed log reduction values of the combinations exceeded by astatistically significant margin the expected log reduction values basedon the performance of the individual antimicrobial agent components ofeach combination.

In addition to the above binary combinations, a less expansive set oftertiary combinations were evaluated. These combinations comprise Bi₂O₃,ZnO and Ag₂CO₃. Concentrations of the individual compounds in thetertiary combinations trialed include Bi₂O₃ at 1% and 2%; at ZnO at 1%and 2%; and Ag₂CO₃ at 0.5%, 1% and 2%.

TABLE 8 Antimicrobial Combination Log Ag₂CO₃ Bi₂O₃ CuO SnO₂ TiO₂ ZnOReduction 2% — 2% — — — 3.9 2% — — 2% — — 3.2 2% — — — — 2% 3.0 2% — — —— 4% 3.3 4% — 2% — — — 4.0 4% — — 2% — — 3.7 4% — — — 2% — 3.6 4% — — —— 2% 3.7 — 2% 2% — — — 1.9 — 2% 2% — — — 2.5 — 2% 4% — — — 3.1 — — 2% 4%— — 2.4 — — 2% — 2% — 2.0 — — 2% — 4% — 2.3 — — 2% — — 4% 3.0 — — 4% 2%— — 4.0 — — 4% — 2% — 3.9 — — 4% — — 2% 3.7 2% 2% — — — — 3.6 2% 4% — —— — 3.0 2% — 4% — — — 3.8 4% 2% — — — — 3.7

Initially, the three compounds were employed at equal concentrations of1% and 2%. As well, combinations were assessed wherein one of Bi₂O₃,Ag₂CO₃ and ZnO was added at 2%, while the other two compounds were addedat 1%. Lastly, trials were undertaken in which two compounds were addedat 2% and the remaining compound at 1%. Results are collected into TABLE9, with log reduction again expressed against bacterial growth on theuntreated sample.

The tertiary combination data show that the three components wereefficacious when present in the ceramic glaze composition at equalconcentrations of 1%. Tertiary antimicrobial agent combinations in whichan increase to 2% of one or both of the Bi₂O₃ concentration and the ZnOconcentration likewise demonstrated efficacy against the bacterialinoculunn.

Antimicrobial activity was greatest for the tertiary combinationcontaining 2% each of Bi₂O₃, Ag₂CO₃ and ZnO. The strength of theantimicrobial activity for this combination exceeds that expected basedon the performance of the component antimicrobial agents individually.

It should be noted that expectations of activity of binary and tertiarycombinations are not reached by simply summing the log reduction valuesfor the separate compounds at the relevant concentrations. Such anapproach may be accurate in cases where the various component compoundsshare a common mechanism of action against the test organism.

TABLE 9 Antimicrobial Agent Log Bi₂O₃ ZnO Ag₂CO₃ Reduction — — — NA 2% —— 0.8 — 2% — 0.4 — — 2% 2.4 2% 2% — 0.7 2% — 2% 1.3 2% — 4% 3.7 — 2% 2%3.0 — 2% 4% 3.7 1% 1% 1% 2.9 2% 1% 1% 2.4 1% 2% 1% 2.3 1% 1% 2% — 2% 2%1% 1.8 2% 1% 2% — 1% 2% 2% — 2% 2% 2% 3.5

However, the literature suggests that bismuth, zinc and silver do notbehave identically in their mechanisms of bacterial attack. Withoutwishing to be bound by theory, in the present instance, zinc is believedto exert its effect by disruption of bacterial respiration and thedelicate equilibrium of metals in the bacterial cell; bismuth isdescribed as inhibiting the bacteria's ability to take up iron; andsilver is believed to act on bacterial proteins involved in nucleic acidreplication.

The results demonstrate that the ceramic glaze disclosed herein showedcommercially acceptable efficacy against Klebsiella pneumoniae relativeto the control. These results are exciting as permitting use ofmaterials in considerably lower amounts than heretofore have beenemployed, especially for those compounds which have previously beenexplored as antimicrobial agents.

The observed results further indicate synergistic actions betweenmaterials, providing enhanced efficacy levels at lower addition amountsof the antimicrobial agents. Decreased addition amounts reduce the costand potential deleterious effect of the compounds in the ceramic glaze.

As well, additional benefits are realized to the environment, in termsof both waste production during manufacture and disposal of the ceramicglaze articles upon termination of their useful product lives.

As noted previously, the antimicrobial ceramic glaze was designed toimpart durable (persistent) and built-in antimicrobial protection to avariety of ceramic articles. Accordingly, the scope of the disclosureincludes ceramic articles that incorporate the present antimicrobialglazing. Such articles include, but are not limited to, toilets, bidets,washbasins, towel rails, soap holders, toilet roll holders, watercontrol fixtures (e.g., hot and cold water handles), and ceramic glazedtiles.

It will therefore be readily understood by those persons skilled in theart that the present composition and methods are susceptible of broadutility and application. Many embodiments and adaptations other thanthose herein described, as well as many variations, modifications andequivalent arrangements, will be apparent from or reasonably suggestedto one of ordinary skill by the present disclosure and the foregoingdescription thereof, without departing from the substance or scopethereof.

Accordingly, while the present composition and methods have beendescribed herein in detail in relation to its preferred embodiment, itis to be understood that this disclosure is only illustrative andexemplary and is made merely for purposes of providing a full andenabling disclosure.

The foregoing disclosure is not intended or to be construed to limit orotherwise to exclude any such other embodiments, adaptations,variations, modifications and equivalent arrangements.

What is claimed is:
 1. A method for manufacturing a glazed ceramicsubstrate having an antimicrobial property, comprising: applying anantimicrobial ceramic glazing composition to a substrate, the glazingcomposition including an antimicrobial composition comprising: a firstantimicrobial agent consisting of Ag₂CO₃; and a second antimicrobialagent that is one of Bi₂O₃, CuO, or ZnO; wherein the first antimicrobialagent is present in the glaze composition at a concentration of about0.5 to about four percent, by weight of glaze composition; and whereinthe second antimicrobial agent is present in the glaze composition at aconcentration of from about one percent to about four percent, by weightof glaze composition; firing the glazing composition to transform theglazing composition into a fired glaze layer; wherein the fired glazedlayer possesses an antimicrobial property.
 2. The method of claim 1wherein the second antimicrobial agent is Bi₂O₃.
 3. The method of claim1 wherein the second antimicrobial agent is ZnO.
 4. The method of claim1 wherein the second antimicrobial agent is CuO.
 5. The method of claim1 wherein the first antimicrobial agent and the second antimicrobialagent are present in the glaze composition at substantially equalconcentrations.
 6. The method of claim 1 wherein the concentration ofthe first antimicrobial agent in the glaze composition is about twopercent; wherein the second antimicrobial agent is one of Bi₂O₃, CuO, orZnO.
 7. The method of claim 1 wherein the concentration of the firstantimicrobial agent in the glaze composition is about four percent; andwherein the second antimicrobial agent is present in the glazecomposition at a concentration of about two percent.
 8. A glazed ceramicsubstrate, comprising: ceramic substrate having a first substratesurface; and a fired ceramic glazing composition disposed on the firstsubstrate surface; wherein the fired antimicrobial ceramic glazingcomposition is a post-fired state of an antimicrobial ceramic glazingcomposition including: a first antimicrobial agent comprised of Ag₂CO₃,and a second antimicrobial agent comprised of Bi₂O₃, CuO or ZnO; whereinthe first antimicrobial agent has a concentration in the ceramic glazecomposition of from about two percent to about four percent, by weightof glaze composition; and wherein the second antimicrobial agent has aconcentration of from about two percent to about four percent, by weightof glaze composition.
 9. The glazed ceramic substrate of claim 8 whereinthe second antimicrobial agent is Bi₂O₃.
 10. The glazed ceramicsubstrate of claim 8 wherein the second antimicrobial agent is CuO. 11.The glazed ceramic substrate of claim 8 wherein the second antimicrobialagent is ZnO.
 12. The glazed ceramic substrate of claim 8 wherein thefirst antimicrobial agent and the second antimicrobial agent are presentin the glaze composition at substantially equal concentrations.
 13. Theglazed ceramic substrate of claim 8 wherein the concentration of thefirst antimicrobial agent in the glaze composition is about two percent;wherein the second antimicrobial agent is one of Bi₂O₃, CuO, or ZnO. 14.The glazed ceramic substrate of claim 8 wherein the concentration of thefirst antimicrobial agent in the glaze composition is about fourpercent; and wherein the second antimicrobial agent is present in theglaze composition at a concentration of about two percent.
 15. A glazedceramic substrate, comprising: ceramic substrate having a firstsubstrate surface; and a fired ceramic glazing composition disposed onthe first substrate surface; wherein the fired antimicrobial ceramicglazing composition is a post-fired state of an antimicrobial ceramicglazing composition including: a first antimicrobial agent comprised ofAg₂CO₃, and a second antimicrobial agent comprised of SnO₂; wherein thefirst antimicrobial agent has a concentration in the ceramic glazecomposition of from about two percent to about four percent, by weightof glaze composition; and wherein the second antimicrobial agent has aconcentration of about two percent by weight of glaze composition.